Gas-operated water heater

ABSTRACT

A gas-operated water heater has a peripheral water tank surrounded by a thermally insulated jacket. A cavity within the peripheral tank contains a central water tank and an annular flue is defined between the two tanks. A burner and a pilot light are situated beneath the tanks the fumes from the burner passing through the annular flue and the fumes from the pilot light passing through a flue in the central tank. The tanks are in fluid connection and cold water may be fed to them and hot water drawn off.

United States Patent Inventors James Rouet Bois-Colombes; Roger Vignes, Paris, France Appl. No. 856,232

Filed Aug. 6, 1969 Patented May 25, 1971 Assignee Service National dit. Gas De France Paris, France Priority Aug. 9, 1968, Dec. 5, 1968 France 162,566 and 176,864

GAS-OPERATED WATER HEATER V 20 Claims, 2 Drawing Figs.

US. Cl 122/16, 122/165 Int. Cl F221) 7/04 Field ofSearch 122/14, 16,

[56] References Cited UNITED STATES PATENTS 1,234,073 7/1917 ONeal et a1 122/165 1,837,597 12/1931 Thomas 122/165X 3,124,] 10 3/1964 Buehl 122/19X Primary ExaminerKenneth W. Sprague Attorney-Hamman and Harpman /6 1 2 E14 i a r a 5 4 f;

2 7 l l a A F 5 2 Sheets-Sheet 1 Patented May 25, 1971 lnvenlor James Rove-t" 9 Buyer V/ynes WW tlorney GAS-OPERATED WATER HEATER The present invention relates to gas-operated water heaters.

in gas-operated water heaters known up to the present time the capacity of the apparatus is dependent on the heat exchange surface between the water and combustion products. if the water is contained in a hot water storage tank, to increase the capacity entailed constructing larger apparatus.

lf the heat exchange surface is increased, heat losses are also increased. At the heat exchange surface of the storage tank the heat losses, in fact, are essentially dependent on the temperature of the heat exchange surface, the temperature and flow of air.

In order to solve the above-mentioned problem, heaters have been proposed which are of a complex design.

Some of these heaters are known as storage heaters when the heating is effected intermittently and water is drawn off from a hot water tank.

When the storage is of low or medium capacity (1 to 6 therms per hour), it may, in a known form, comprise a tank serving as water reservoir and a central flue, the walls of which constitute a heat exchanger. Deflectors disposed in the central flue over the entire height of the tank, create a loss of head and throttle the combustion products against the wall of the tank in order to assist heat exchange.

In order exchange obtain a large heat exchange surface in a storage heater of this type, it is necessary to provide a flue of large diameter, thus increasing the amount of combustion products which do not participate in the exchange of heat. During the flow of the combustion products through the flue towards the main outlet of the storage heater, only a thin cylindrical layer is in fact in contact with the wall of the tank and participates efiectively in the transfer of heat;- this consequently entails poor utilization of the heat of the combustion products.

With regard to large capacity storage heaters (more than 7 therms per hour), it was not practicable to provide them with a single central flue without reducing proportionally the water capacity for a given outside diameter of the tank. It was therefore found necessary to equip the storage heater with a multitubular heat exchanger.

A disadvantage of this type of storage heater is that is it expensive and difficult to manufacture because of the many welds required to join together the various pipes in the pipe system. In addition, it is difficult to provide the welds with effective protection against corrosion.

Additionally, there is also the disadvantage that to increase the capacity of the storage heater it is necessary to increase its exchange surface and therefore its dimensions.

As a result, the internal heat losses when the burner is not working become prohibitive, particularly because of the large internal heat exchange surfaces; similarly, it is not possible to eliminate the loss of heat produced by the pilot light.

Since in fact the pilot light burns permanently, its consumption of gas is far from negligible; consequently, in known arrangements, the heat produced by the pilot light is poorly used and reduces the efficiency of the storage heater. Finally, as the combustion products of the pilot light are diluted in the flue, the thermal draught is increased, thus increasing heat losses through the central flue.

According, therefore, to the present invention there is provided a gas-operated water heater comprising a peripheral water tank, a thermally insulated jacket surrounding said peripheral tank, means defining a cavity in the peripheral tank, a central water tank disposed in said cavity coaxially with said peripheral tank, an annular flue deflned between said peripheral tank and central tank, a burner disposed beneath said tanks, a mixer adapted to supply the burner with an air and gas mixture whereby, in operation, combustion products from said burner pass through the annular flue, pilot light means for producing a pilot light, cold water inlet means and hot water outlet means for said tanks, and a pilot flue passing through said central tank for receiving the combustion products of said pilot light.

The apparatus may include a passage disposed beneath said tanks for admission secondary combustion air. Preferably a cold water inlet pipe at the base of the peripheral tank, said cold water inlet pipe being so disposed that, in operation, cold water enters the peripheral tank tangentially to the bottom thereof to provide a helical flow of cold water in the peripheral tank.

One advantage of the above apparatus is that the heat exchange surface and the water storage capacity are considerably increased because of the presence of the central tank.

Another advantage is that the central stream of combustion products is completely eliminated and an annular flue to provided, this thermal efficiency is high.

Thus the cold water inlet means may have an end in said peripheral tank which is elbowed. The cold water following the helical path produces a layer of cold water immediately below the hot water in the peripheral tank, without entailing any substantial mixing of the cold and hot water; it is thus possible to run off up to percent of the hot water capacity of the heater at a temperature close to maximum.

Because the burner has a very large air supply, the combustion of the gases requires only a small addition of additional said thermally insulated jacket may enclose said tanks and burner in an airtight manner, an air admission pipe and outlet pipe being provided, said air admission pipe being disposed at substantially the same level as the outlet pipe and being in communication with said outlet pipe only via the annular flue. Thus additional air to the burner is restricted by the thermally insulated jacket.

Preferably the pilot flue is coaxial with the central tank and at one end is disposed substantially adjacent the pilot light means. Thus the combustion products of the pilot light are discharged exclusively through the pilot flue and, excessive cooling of the central tank is avoided since it takes up heat from the combustion products produced by the pilot light.

The pilot light and the outletpipe are preferably so dimensioned that the heat produced by the pilot light is slightly lower than the heat losses of the apparatus during periods when the burner is not working, the slight difference between the heat produced by the pilot light and the heat loss of the apparatus is intended to take into account climatic variations in the environment of the apparatus.

In certain of the prior art devices it has been found very difficult to keep the heat losses at a low value when the capacity of the water heater is increased. However, with the present case it has been found that the heat losses suffered by the water heater during the inoperative period, between two successive operations of the burner, can account for from 45 to 50 percent of the total losses, which are very low compared with the prior art.

The air admission pipe may connect with an annular space between the peripheral tank and the thermally insulated jacket, an annular diaphragm being provided fixed to the top end of the peripheral tank and the said outlet pipe. Consequently, the heat losses are further reduced since the hot fumes of the pilot light produce only an extremely slight thermal draught and the air passing through the annular flue during the inoperative period of the burner is practically always at the same temperature as that of the water.

As the thermal draught is very weak during the inoperative periods of the burner, the capacity of the apparatus can be increased without fearing a proportional increase of the thermal draught.

Another advantage of the present invention which is significant is that the apparatus is silent, because the combustion noise of the burner is mufi'led.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which:

FIG. 1 is an axial section of one embodiment of a gasoperated water heater according to the present invention, and

FIG. 2 is an axial section of a second embodiment of a gasoperated water heater according to the present invention.

Referring first to FIG. 1 a gas-operated water heater comprises a thermally insulated jacket I in which there is disposed a peripheral hollow cylindrical tank 2 defining a central flue 3. Inside the flue 3 there is disposed, concentrically with the peripheral tank 2 by means of pipes 5 and 6, one of which 5 connects the top of the central tank 4 to the top of the peripheral tank 2, while the other pipe 6 connects the bottom of the central tank 4 to the bottom of the peripheral tank 2. An annular space 7 is defined between the outer wall defined between 4a of the central tank 4 and the inner wall 2a of the peripheral tank 2. This annular space 7 is so dimensioned that the flow of fumes or combustion products permits optimum heat exchange over the entire axial length of the central tank 4. A pilot flue or pipe 8 for the discharge of fumes produced by a pilot light 9 of a burner unit 10 extends through the central tank 4 preferably along its axis.

The provision of a central tank 4 in the flue 3 makes it possible for the heat exchange surface to be considerably increased. The combustion products produced by the burner unit 10 are in fact throttled into the annular space 7 and fully participate in the heat exchange between the combustion products and the inner wall 2a of the peripheral tank 2 and the outer wall 4a of the central tank 4. The walls 2a and 4a constitute the heat exchange surfaces and the area of heat exchange surfaces is almost twice that of the prior art case in which the central tank 4 is omitted. The walls 2a and 4a are preferably smooth.

Moreover, without increasing the dimensions of the apparatus, the hot water storage capacity is substantially increased over the prior art case since the central tank 4 also contains water which is brought to a temperature approximately equal to that of the water contained in the peripheral tank 2. In one example of embodiment the annular space 7 has a radial width 1 of about mm., the surface of the outer wall 40 constitutes 40 percent of the total heat exchange surface, and the volume of the central tank 4 constitutes about 15 percent of the total of volume used for storing hot water.

The bottom end 8a of the pipe 8 leads from the central tank 4 into a combustion chamber 9a and is situated near the pilot light 9, while the top end 8b is partially closed by a draught regulating device, such as a diaphragm 11, and leads into an outlet pipe 12 communicating with the flue 3.

The pipe 8 has a heat exchange surface such that the heat exchange with the water and the heat produced by the pilot light 9 is optimized. Obviously, the diaphragm ll situated near the top end 8b of the pipe 8 could be replaced by a constriction provided in the pipe 8 or by an appropriate internal spiral heat exchanger.

A diaphragm 13, for example, is provided in the lower end of the outlet pipe 12 to choke the combustion products from the burner unit 12 in order to obtain a higher heat exchange rate.

A cold water supply pipe 14 passes through the top part of the peripheral tank 2 and has a bevelled end 15 which opens into the peripheral tank 2 near its base, so that the water flows tangentially to the base of the peripheral tank 2 and the cold water rises in said tank helically. It is obvious that the supply pipe 14 may either pass through the peripheral tank from top to bottom, as indicated in FIG. ll, or lead into said tank directly from the base of the peripheral tank 2.

The helical flow has the result that as the peripheral tank 2 is filled with cold water from the'base, the hot water already in the peripheral tank 2 is not disturbed and is merely moved towards the top of the peripheral tank.

In other words, mixing does not occur between the hot water and the cold water,and the two layers of water remain separated. Thus, the hot water run off is practically always at the same temperature.

The peripheral tank 2 is also provided at its top end with a pipe 16 for the discharge of hot water, and at its bottom end with a water drain pipe 17.

In this embodiment the base of the burner unit 10 has only a small passage for the admission of secondary air for combustion. The burner unit 10, comprises a fluidtight casing 18, having aplate 19 provided on its periphery with a plurality of apertures 20 therein and a mixer 2l-adapted to supply the burner unit 10 with a mixture of gas and primary airfor combustion; secondary air for combustion is admitted through a small central cavity 22 in the casing 18 this cavity containing the pilot light 9. The casing I8 is fixed in a fluidtightmanner on the thermally insulated jacket 1.

In addition, baffles 23 are provided in the casing 18 and are disposed in the flow path of the gas primary air from the mixer 21, in order to distribute this mixture more effectively. The burner unit 10 not only makes it possible to obtain very hot and only slightly diluted combustion products, but also enables the heat losses through the flue 3 during periods when the burner unit 10 is not working to be reduced substantially, because entry of air is limited to the small cavity 22.

A thermostat 24, preferably disposed at a distance between one-quarter and one-third of the total height of the apparatus from the burner unit It), enables relighting of the burner unit 10 to occur at sufficiently long intervals to obtain high practical efficiency, even for series of repeated small withdrawals of hot water; this delay in ignition does not constitute a disadvantage because of the capacity of the burner unit 10. This is in contrast to known heaters in which the thermostat is situated near the burner unit 10 in that the burner unit is frequently lit and consequently gas consumption is high. The arrangement of FIG. 1 makes it possible to avoid the risk of overheating.

The capacity of the burner unit 110, the effectiveness of heat exchange, and the water storage capacity of the peripheral and central tanks make it possible to place at the disposal of the user large amounts of hot water, such as for example are necessary for supplying several bathrooms. The heating of enough water for a bath, for example, with a heater according to the present invention, which has a water capacity of about litres and a burner unit efficiency of 17 therms per hour, and in which the peripheral tank 2 has an outside diameter of 38 cm., an inside diameter of 18 cm., and a height of 98 cm., while the central tank 4 has an outside diameter of 15 cm. and a height of 79 cm., is effected in about 10 minutes with a continuous flow, and it is then possible to run off hot water at 65 C. at a rate of about 5 litres per minute.

FIG. 2 illustrates a second embodiment of a gas-operated water heater according to the present invention. Identical or similar parts to those illustrated in FIG. 1 are given the same references and will not be further described.

As can be seen in FIG. 2, the top end 11 of the pipe 8 is situated inside the outlet pipe 112 and is approximately at the level of an air intake 26. This arrangement prevents fumes from the pilot light from producing a harmful thermal draught during periods when the burner unit 10 is not lit or is inoperative.

According to the second embodiment of the invention, the base and sidewall of the thermally insulated jacket l are closed and in airtight manner with an air admission pipe 25. The air intake 26 is situated substantially at the same level as the outlet mouth 12a of the outlet pipe 12, and is in communication with the outlet pipe ll2 only by way of the flue 3, and the annular space 7.

In a modified form of this embodiment, the air admission pipe 25 may lead into the thermally insulated jacket 1 at its bottom end. This, however, does not make it possible for the air to be heated before it reaches the burner 10. This being so, the air intake 26 must be situated at a lower level than that of the outlet pipe 12, in order to obtain a siphon effect for combustion air.

It is advantageous to provide a small annular space 27 between the outside wall 2b of the peripheral tank 2 and the inside wall of the thermally insulated jacket 1, to lead the bottom end of the air admission pipe 25 into the top portion of the jacket I and to fix the outer edge of the diaphragm l3, constituted by an annular disc, on the opt end of the peripheral tank 2, and likewise to fix, in a fluidtight manner, the bottom end of the outlet pipe 12 on the diaphragm 13 adjacent to the inside edge.

By this arrangement, an air admission circuit is formed which together with the flue 3, annular space 7 and the outlet pipe 12 produces a geometrically and thermally balanced siphon during the inoperative regulation periods of the burner unit 10. During these regulation periods in fact, the columns of air situated on the one hand in the admission pipe 25 and the annular space 27 and on the other hand in the flue 3, annular space 7 and the outlet pipe 12 rapidly become balanced, because when the burner unit is extinguished all the walls inside the apparatus soon achieve a mean temperature, for example of the order of 65 C., and the fresh air needed for the operation of the pilot light 9 moves downwards only with a very slight flow through the annular space 27. In this manner the thermal draught during the inoperative periods is very slight, and consequently the thermal losses are practically reduced to zero during these periods.

In the drawing, the air admission pipe 25 is provided at the side of the outlet pipe 12.

In an advantageous modification of the invention, which is not illustrated, the admission pipe 25 and the outlet pipe 12 are disposed concentrically in relation to one another, the annular space between the two pipes being in communication on the one hand with the air intake 26 and on the other hand with the annular space 27 between the peripheral tank 2 and the thermally insulated jacket 1. In this manner the balance between the two columns of air, which was referred to previously, is achieved even more quickly.

The outlet pipe 12 passes in a fluidtight manner through the top wall la of the thermally insulated jacket 1 and leads horizontally, by means of an elbow 12b, into a draught box 28 partially open to the atmosphere at its bottom part, through an orifice 28a, and is provided at its top part with a fume duct 29 connected to atmosphere. A deflector 30, disposed in front of the top part of the outlet mouth 12a of the elbow 12b and directed towards the orifice 28a of the draught box 28, prevents reverse draughts due to puffs of winds from penetrating into the flue 3 and extinguishing the pilot'light 9 and even the burner unit 10.

The air intake 26 is disposed substantially at the same level as the bottom part of the deflector 30. Like the draught box 28, this air intake may open into the atmosphere, that is to say outside an apartment, so that the apparatus is perfectly fluidtight in relation to the room in which it is situated and is very silent when the burner unit 10 is operating. In addition, the apparatus is consequently absolutely safe even in the event of faulty operation.

According to another feature of this embodiment of the invention, the mixer 21 supplying the burner unit 10 with a mixture of primary air and gas is in direct communication through orifices 31 with a space bounded by the bottom end of the thermally insulated jacket 1, so that it draws in air for combustion through the annular space 27. This air is generally preheated by the previously mentioned arrangements. Here again, it is not possible for cold air to enter at the mixer 21, which could entail heat losses during inoperative periods. The pipe supplying gas passes in a fluidtight manner through the jacket 1 before reaching the mixer 21. In addition, the secondary air necessary for correct combustion of the gas is also supplied through the annular space 27. An access door 32 is provided in the thermally insulated casing opposite the burner unit 10, this door normally being closed in a fluidtight manner.

According to an additional feature of the second embodi' ment of the invention, a crown-shaped head burner 10a is disposed at the base of the combustion chamber 9a, that is to say in the extreme bottom portion of the annular space 7. It is thus ensured that the combustion products are always discharged through the annular space 7 and the flue 3 and never reach the air admission pipe 25, where they could obstruct the admission of fresh air.

The results obtained with an apparatus according to the second embodiment of the invention testify to a surprising technical improvement. The overall heat losses of-a known apparatus resulted in a water temperature drop in the order of C. with 10 hours interval between two successive operations of the burner with a mean water temperature of 65 C. whereas the present invention resulted in a temperature drop of only 8 C. in 10 hours, both apparatus being of the same capacity. In addition, the previously mentioned advantages have the consequence of considerable economy with regard to gas consumption.

Although the second embodiment of the invention has been described in connection with an apparatus comprising two concentric water tanks, the various means proposed by the invention are also applicable to heaters having a single water tank, or else having more than two concentric tanks.

lclaim:

1. A gas-operated water heater comprising a peripheral water tank, a thermally insulated jacket surrounding said peripheral tank, means defining a cavity in the peripheral tank, a central water tank disposed in said cavity coaxially with said peripheral tank, an annular flue defined between said peripheral tank and central tank, a burner disposed beneath said tanks, a mixer adapted to supply the burner with an air and gas mixture whereby, in operation, combustion products from said burner pass through the annular flue, pilot light means for producing a pilot light, cold water inlet means and hot water outlet means for said tanks, and a pilot flue passing through said central tank for receiving the combustion products of said pilot light.

2. An apparatus as claimed in claim 1 including passage disposed beneath said tanks for admission secondary combustion air.

3. An apparatus as claimed in claim 1 in which said thermally insulated jacket encloses said tanks and burners in an airtight manner, an air admission pipe and an outlet pipe being provided, said air admission pipe being disposed at substantially the same level as the outlet pipe and being in communication with said outlet pipe only via the annular flue.

4. An apparatus as claimed in claim 1 in which the pilot flue is coaxial with the central tank and at one end is disposed substantially adjacent the pilot light means.

5. An apparatus as claimed in claim 1 in which the cold water inlet means comprises a cold water inlet pipe at the base of the peripheral tank, said cold water inlet pipe being so disposed that, in operation, cold water enters the peripheral tank tangentially to the bottom thereof to provide a helical flow of cold water in the peripheral tank.

6. An apparatus as claimed in claim 1 in which the cold water inlet means has an end in said peripheral tank which is elbowed.

7. An apparatus as claimed in claim 1 in which the thermostat is disposed at least partially in the peripheral tank at a height between one-quarter and one-third of the total height from the bottom of the tank.

8. An apparatus as claimed in claim 1 in which the central tank is connected to the peripheral tank by at least two pipes one of which is disposed at the top end of the peripheral tank and the other at the bottom end.

9. An apparatus as claimed in claim 2 in which the burner has a fluidtight casing having a central cavity defining said passage, the pilot means being disposed partially within the cavity and said casing being connected to said mixer to supply air/gas mixture to the burner.

10. An apparatus as claimed in claim 4 in which a draughtregulating means are provided in the pilot flue and the outlet pipe.

11. An apparatus as claimed in claim 3 in which the air admission pipe connects with an annular space between the peripheral tank and the thermally insulated jacket, an annular diaphragm being provided fixed to the top end of the peripheral tank and the said outlet pipe.

12. An apparatus as claimed in claim 11 in which the air admission pipe is disposed adjacent the outlet pipe.

13. An apparatus as claimed in claim 11 in which the air admission pipe and outlet pipe are disposed concentrically relative to each other and defining an annular gap therebetween, said annular gap being in communication with an air intake and the said annular space.

M. An apparatus as claimed in claim 11 in which the outlet pipe passes in a fluidtight manner through the thermally insulated jacket, leads horizontally through an elbow into a draught box which is partially open at its base to atmosphere by means of an orifice and at its top is provided with a fume pipe connected to atmosphere.

15 An apparatus as claimed in claim 14 in which a deflector is disposed in the said elbow.

16. An apparatus as claimed in claim 15 in which the air intake is disposed substantially at the same level as the bottom I0 portion of the deflector.

17. An apparatus as claimed in claim 9 in which the casing 

1. A gas-operated water heater comprising a peripheral water tank, a thermally insulated jacket surrounding said peripheral tank, means defining a cavity in the peripheral tank, a central water tank disposed in said cavity coaxially with said peripheral tank, an annular flue defined between said peripheral tank and central tank, a burner disposed beneath said tanks, a mixer adapted to supply the burner with an air and gas mixture whereby, in operation, combustion products from said burner pass through the annular flue, pilot light means for producing a pilot light, cold water inlet means and hot water outlet means for said tanks, and a pilot flue passing through said central tank for receiving the combustion products of said pilot light.
 2. An apparatus as claimed in claim 1 including passage disposed beneath said tanks for admission secondary combustion air.
 3. An apparatus as claimed in claim 1 in which said thermally insulated jacket encloses said taNks and burners in an airtight manner, an air admission pipe and an outlet pipe being provided, said air admission pipe being disposed at substantially the same level as the outlet pipe and being in communication with said outlet pipe only via the annular flue.
 4. An apparatus as claimed in claim 1 in which the pilot flue is coaxial with the central tank and at one end is disposed substantially adjacent the pilot light means.
 5. An apparatus as claimed in claim 1 in which the cold water inlet means comprises a cold water inlet pipe at the base of the peripheral tank, said cold water inlet pipe being so disposed that, in operation, cold water enters the peripheral tank tangentially to the bottom thereof to provide a helical flow of cold water in the peripheral tank.
 6. An apparatus as claimed in claim 1 in which the cold water inlet means has an end in said peripheral tank which is elbowed.
 7. An apparatus as claimed in claim 1 in which the thermostat is disposed at least partially in the peripheral tank at a height between one-quarter and one-third of the total height from the bottom of the tank.
 8. An apparatus as claimed in claim 1 in which the central tank is connected to the peripheral tank by at least two pipes one of which is disposed at the top end of the peripheral tank and the other at the bottom end.
 9. An apparatus as claimed in claim 2 in which the burner has a fluidtight casing having a central cavity defining said passage, the pilot means being disposed partially within the cavity and said casing being connected to said mixer to supply air/gas mixture to the burner.
 10. An apparatus as claimed in claim 4 in which a draught-regulating means are provided in the pilot flue and the outlet pipe.
 11. An apparatus as claimed in claim 3 in which the air admission pipe connects with an annular space between the peripheral tank and the thermally insulated jacket, an annular diaphragm being provided fixed to the top end of the peripheral tank and the said outlet pipe.
 12. An apparatus as claimed in claim 11 in which the air admission pipe is disposed adjacent the outlet pipe.
 13. An apparatus as claimed in claim 11 in which the air admission pipe and outlet pipe are disposed concentrically relative to each other and defining an annular gap therebetween, said annular gap being in communication with an air intake and the said annular space.
 14. An apparatus as claimed in claim 11 in which the outlet pipe passes in a fluidtight manner through the thermally insulated jacket, leads horizontally through an elbow into a draught box which is partially open at its base to atmosphere by means of an orifice and at its top is provided with a fume pipe connected to atmosphere.
 15. An apparatus as claimed in claim 14 in which a deflector is disposed in the said elbow.
 16. An apparatus as claimed in claim 15 in which the air intake is disposed substantially at the same level as the bottom portion of the deflector.
 17. An apparatus as claimed in claim 9 in which the casing of the burner and the mixer are disposed within the thermally insulated jacket.
 18. An apparatus as claimed in claim 17 in which the mixer has an air intake orifice which communicates directly with the bottom of the thermally insulated jacket.
 19. An apparatus as claimed in claim 17 in which a fluidtight access door is provided in the thermally insulated jacket adjacent the burner.
 20. An apparatus as claimed in claim 3 in which the top end of the pilot flue is disposed within the outlet pipe. 